Blue-Green Algae (Cyanobacteria) Profile and How to Remove

Blue-Green Algae (Cyanobacteria) Profile and How to Remove

Cyanobacteria, commonly known as “Blue-green algae or BGA”, can be a common issue in aquariums. Generally, cyanobacteria appear as slimy, greenish-blue, or blackish mats that cover surfaces in the tank.

Cyanobacteria are very resilient and can be hard to deal with since they require a minimal set of conditions for survival in the tanks. Nonetheless, there are several well-established methods (such as special medicine, blackout, manual removal, etc.) for combating them.

Now, let’s have a look at the description, common causes, and some ways to avoid or remove Blue-green algae from the tanks.

What Are Blue-Green Algae?

Blue-Green Algae (Cyanobacteria) Profile and How to Remove - What Are Blue-Green AlgaeIn reality, Blue-green algae are not true algae. They are actually a group of photosynthetic bacteria that occupy a unique place between bacteria and algae. Thus, they are called cyanobacteria.

Note: The name “cyanobacteria” is derived from two components: “Cyano-“, which means “blue”, and “bacteria”.

Like algae, cyanobacteria are among the oldest known organisms on Earth (approximately 3.5 billion years old) and played a crucial role in shaping the planet’s early atmosphere by producing oxygen through photosynthesis.

Based on morphological characters and molecular analyses, there are around 8000 cyanobacteria species.

They can be found in a diverse range of environments, including freshwater, brackishwater, saltwater, and terrestrial ecosystems. Cyanobacteria range in size from 0.5 to 60 micrometers.

Despite the fact that from a scientific standpoint, they are not algae, the term “Blue-green algae” has become so commonly used in aquarium circles that I’ll still refer to them as such.

How to Identify Blue-Green Algae in the Tank

Although there are many species of cyanobacteria, they generally share some common characteristics such as:

  • Color. Blue-green algae usually have a dark green or blue-shimmering (in rare cases even reddish-brown) appearance.
    Interesting fact: The color of Blue-green algae depends on the spectrum of light they absorb.
  • Texture. Cyanobacteria often appear as slimy or gelatinous masses.
  • Form. Blue-green algae are composed of individual cells, but these cells can aggregate to form visible colonies. In aquariums, they can form thin layers or patches on any surface, such as substrate, decorations, rocks, equipment, and plants.
  • Smell: Many types of blue-green algae emit an unpleasant odor (swampy), especially when disturbed.

Blue-Green Algae

Reproduction of Blue-Green Algae

Blue-green algae reproduce through a process called binary fission (in simple words by splitting in two), which is a form of asexual reproduction.

  • The process begins with the replication of the genetic material (DNA) within the cell.
  • As the genetic material replicates, the cyanobacterial cell gets elongated and starts to separate the duplicated DNA molecules.
  • The cell divides into two new cells, each with a complete set of DNA.

Under favorable conditions, Blue-green algae can reproduce at explosive rates, doubling their population in a matter of 6-12 hours. Given such exponential growth, it is possible to notice the formation of a quite visible layer in the form of a mat as early as the next day.

Interesting fact: China and South America have started to explore the process of converting Cyanobacteria into flour so many food-related issues could be resolved.

Causes of Blue-Green Algae

Considering the wide range of cyanobacteria species, it is quite difficult to provide an exact list of causes. Through lots of discussions and debates, aquarists came to the assumption that the primary factors include:

  1. Imbalanced nutrients. The improper ratio of nitrates and phosphates (recommended ratio is 10:1), or their high levels provide cyanobacteria with the resources they need to thrive.
    Overstocking, overfeeding, accumulation of organic matter due to infrequent maintenance or improper equipment operation, and excessive doses of plant fertilizers – all these things may contribute to nutrient buildup.
  2. Excessive Light. The fact that the growth of cyanobacteria is stimulated by lighting (they are photosynthetic) is another major reason for its appearance in the tank. Intensive lighting (such as direct sunlight) can trigger cyanobacteria growth. The duration of light exposure is also important.
  3. Lack of nitrogen. One of the main causes of Blue-green algae is also a lack of nitrogen (10<).
  4. Inadequate water circulation. Stagnant or low water circulation areas in the tank can create pockets where cyanobacteria can settle and grow. Additionally, compact substrates (like sand) that do not allow oxygen to circulate are the perfect place for BGA.
  5. Poor water quality: Suboptimal water parameters, such as fluctuating pH or temperature. It was also noticed that blue-green algae prefer alkaline water (pH>7.5).
  6. Unstable systems: Disruptions to the beneficial bacterial balance (new tank setups or mini-cycles) in the tank can also benefit cyanobacteria.

Obviously, many of these factors are interrelated, and in most cases, the appearance of Blue-green algae is caused not by a single factor, but rather by their combination.

Are Blue-Green Algae Harmful to Fish or Shrimp?

Although most cyanobacteria in aquariums are safe for fish, shrimp, and other creatures certain types (such as Microcystis, Anabaena, etc.) can release toxins that may affect the health and well-being of your aquatic inhabitants.

Fortunately, this occurs very rarely. Therefore, the presence of Blue-green algae in the tank is not a reason to panic.

Are Blue-Green Algae Harmful to Plants?

Blue-Green Algae (Cyanobacteria) Profile and How to Remove - on plantsThe short answer is yes.

Blue-green algae have a remarkably fast growth rate and can cover virtually any surface. As a result, plants, especially slow-growing ones, struggle to compete for nutrients. Additionally, they are unable to receive the light necessary for photosynthesis which will definitely hinder the growth and health of the plants.

How to Remove Blue-Green Algae

Dealing with this type of algae should start as soon as you spot them in your tank. Otherwise, within a matter of days, your entire tank can be covered in unattractive and unpleasant-smelling “slime.”

There are several effective methods for combating these bacteria – manual cleaning and the use of chemical treatments are probably the most popular ones.

Biological methods of control do not work with Blue-green algae.

Except for Moina, in freshwater tanks, there are no fish or invertebrate species that can eat it. I repeat – nothing will eat BGA. 

Although some people say that Ramshorn snails may eat cyanobacteria in aquariums, neither I nor my friends have ever witnessed such behavior.

I think this is one of those things where someone writes something on the internet, and then it just starts getting repeated by everyone without any real basis.

1. Manual cleaning

  1. Use a soft-bristle brush to gently scrub the cyanobacterial mats off surfaces. Be cautious not to damage plants, however, in cases of severe contamination, you may have to trim and remove affected parts.
  2. All decorations, driftwood, etc. should be thoroughly rinsed under running water.
  3. Use a siphon to vacuum the substrate and open areas with cyanobacteria. This also prevents its spores from settling elsewhere.
  4. Do water change to remove any excess nutrients.

Although Blue-green algae come off easily, in many cases, they may return after a few weeks or even days.


2. Fertilization

Cyanobacteria often appear due to either too low (10<) or too high nitrogen levels (> 50).

Therefore, the first step is to determine what is the case for you. If you have low nitrates, then adding fertilizers can help with this.

You should notice changes within a week.


3. Chemical treatments

Blue-Green Algae (Cyanobacteria) Profile and How to Remove - Chemical treatmentsCyanobacteria can be effectively eliminated through the use of certain aquarium medications or antibiotics.

Be prepared for a mass die-off of Cyanobacteria and an increase in ammonia and nitrite levels in the water, accompanied by a sudden release of toxins.

Therefore, clean the mechanical filter daily and remove as much Cyanobacteria as possible.


3.1 Erythromycin

Blue-Green Algae (Cyanobacteria) Profile and How to Remove - Erythromycin

  1. Do a large water change (30-50%) and manually remove as much Blue-green algae as possible.
  2. Turn off filtration (erythromycin may harm beneficial bacteria).
  3. Remove carbon during treatment.
  4. Turn off UV Sterilizer.
  5. Maintain adequate water flow or aeration
  6. Add 500 mg of erythromycin per 30 gallons (100 liters) of aquarium water.
    Note: It is important to add it specifically at night when the lights are turned off. Do not add it during the day.
  7. Next day manually remove dead Blue-green algae.
  8. Do another large water change.
  9. Repeat the dose of erythromycin.
  10. Do another large water change.
Diana Walstad also wrote in her book (The Ecology of the Planted Aquarium). At the recommended dosage of 200mg/10 gallons, erythromycin won’t affect your plants.

3.2 ADA Phyton Git Plus

Blue-Green Algae (Cyanobacteria) Profile and How to Remove - ADA Phyton Git PlusThis is a premium brand in aquascaping and Nature Aquaria. This is a plant supplement, primarily composed of plant extracts, containing antiseptic components. 

It is effective in eliminating Black beard algae and blue-green algae. 

  1. Using a syringe, dose Phyton Git directly where the BGA were amassing.
  2. Apply aeration.
  3. Keep lighting turned off for approximately 3 days.
  4. After 3 days, change 30-50% of the aquarium’s water and remove any remaining cyanophyta.

3.3 Fritz Slime Out

Blue-Green Algae (Cyanobacteria) Profile and How to Remove - Fritz Slime OutFollow the manufacturer’s instructions. Fritz Slime Out helps eliminate stains caused by cyanobacteria outbreaks.

It works effectively against various cyanobacteria strains and it can be used safely with invertebrates, plants, and all fish species.

Fritz Slime Out is suitable for both freshwater and saltwater setups.

3.4 Tetra AlguMin Plus

Blue-Green Algae (Cyanobacteria) Profile and How to Remove - TetraAlguMin PlusTetraAlguMin Plus is a liquid treatment designed to combat algae, containing an effective algicide called monolinuron, which inhibits photosynthesis in Blue-green algae.

The treatment starts working almost instantly and does not affect biofiltration.

According to the manufacturer, when used at recommended dosages, it poses no harm to snails and shrimp.

3.5 Tetra AlgoStop depot

Blue-Green Algae (Cyanobacteria) Profile and How to Remove - Tetra AlgoStop depotUnlike previous medicine, Tetra AlgoStop depot is a slow-dissolving tablet designed for prolonged control of algae growth.

It effectively eliminates various types of algae in freshwater aquariums, including Blue-green algae, while also preventing their further growth.

It contains a highly active, long-lasting anti-algae substance. The tablets gradually dissolve over the course of a few weeks, enabling effective control even against persistent types of algae.

Tetra AlgoStop depot is safe for all aquarium inhabitants, plants, and microorganisms when used according to the provided instructions.

Note: Filtration using activated carbon reduces the effectiveness of the product. All UV sterilizers must be deactivated during the application of this treatment as well.

3.6 Ultralife Blue-Green Slime Stain Remover

Blue-Green Algae (Cyanobacteria) Profile and How to Remove - Ultralife Blue-Green Slime Stain RemoverThis medicine has been specifically formulated to combat the problem of Blue-green algae.

There is no need to change the water or filtration system, making this an impressively practical solution that can be accomplished in minutes.

The safe formula is harmless to fish, invertebrates, and nitrifying bacteria. However, it may affect pH and oxygen levels. So, it is important to increase oxygen due to treatment.

3.7 Hydrogen Peroxide and Spot Treatment

Hydrogen peroxide is an efficient way to battle BGA. The main downside though is that it will be necessary to directly treat the areas affected by cyanobacteria.

shrimp tank Hydrogen PeroxideDosage: Hydrogen peroxide is dangerous to animals. It can burn their gills. So, you have to understand the danger and take responsibility! If you do it, you do it at your own risk!

  1. Turn the filter off!
    Note: We do that so that beneficial bacteria will not be affected.
  2. Turn the air pump off.
    Note: We do that to have very little to no water movement. It will let the hydrogen peroxide be around the algae longer.
  3. Turn the lights off.

Note: The decomposition of H2O2 happens much faster in the presence of light.

  1. Fill a syringe with 3% Hydrogen peroxide, (3ml for every 1 gallon or 4 liters),
  2. Apply it directly to cyanobacteria.
  3. Avoid hitting any plants, shrimp, or fish in the process! Most fish and shrimp are pretty curious and can come to the treatment zone and see what is happening! It will harm them!
  4. Next, in a few hours, manually remove dying algae.
  5. Turn the filter on.
  6. Next day, do your weekly water change.

Potential problems: Overdose Hydrogen peroxide will harm the shrimp and fish. Their gills are very sensitive. Some plants like MossesVallisneriaBucephalandra, etc. can melt. Many other plant species usually respond well.

4. Darkening the Tank

Cyanobacteria are photosynthetic. They thrive in well-lit conditions. Thus, adjusting the lighting duration and intensity can help discourage its growth. Consider reducing the lighting period or investing in a timer to regulate the light cycle.

Potential problems:

  • It can harm plants that require a lot of light but they should survive.

Blue-Green Algae (Cyanobacteria) Profile and How to Remove blackoutHow to do a Blackout:

  1. If you have fish, feed them a regular amount of food. Do not overfeed. If it is a shrimp tank, you may even skip this step.
  2. Turn off the CO2.
  3. Do not add fertilizers during the blackout.
  4. Increase the oxygen supply. Install a new air pump if needed.
  5. Cover the tank with a thick blanket. It must be completely dark in the tank. Cyanobacteria can survive even if there is a little bit of light.
  6. Leave the tank this way for at least 3-4 days.
  7. After the blackout, open up half of the tank and leave it like this for 30 minutes. Let your fish adapt.
  8. Remove the blanket completely and wait another 30 minutes before switching the aquarium light on.
  9. Do a water change.
  10. Manually remove BGA.
  11. Change your light timer to 4 – 5 hours for the next week.


An interesting feature of Cyanobacteria is that they can store glycogen, and when there is no light, they can survive by using the carbohydrates they have accumulated inside them, which means they only break down glycogen. 

How to Prevent Blue-Green Algae

  1. Do not overfeed. Reduce feeding. Make sure that excess food is removed promptly. Vacuum the substrate to remove uneaten food.
  2. Control nitrogen. It should not be too low (10<) or too high (> 50).
  3. Improve water circulation. Adjust aquarium equipment to improve water flow and prevent stagnant areas where cyanobacteria can thrive.
  4. Do not overstock. Always ensure that the number of animals in the tank matches its volume.
  5. Adjusting light. Blue-green algae are photosynthetic. Lighting directly affects their growth rate.
  6. Increase oxygen levels. High oxygen levels in the water suppress cyanobacteria.

When plants or algae make food from light, they also create some strong chemicals like superoxide and peroxide. These can be bad for cells, but plants have special helpers (enzymes) that change them into harmless water. Cyanobacteria do not have these helpers, so they can’t handle the strong chemicals. That’s why they grow well in dirty and low-oxygen water.

According to the ADA

“Poor filtration or inadequate substrate condition create favorable conditions for the appearance of blue-green algae. Therefore, if blue-green algae emerge, the first step is to open and inspect the filter, cleaning the media to remove clogs and ensure sufficient oxygen supply for bacteria. After this, introduce a bacterial culture into the aquarium and consider aerating the tank at night. When the bacterial culture in the filter is healthy, the growth of blue-green algae becomes more challenging. Deterioration of the substrate condition can also be a cause of blue-green algae.”

In Conclusion

The appearance of cyanobacteria in the aquarium can be an unpleasant surprise for any aquarium enthusiast. These bacteria, much like algae, are always present in our tanks – it is only a matter of the conditions that lead to their rapid growth.

That is why it is crucial to address the underlying causes of their growth. Improving water quality, adjusting lighting, maintaining proper filtration, and managing nutrient levels are essential for long-term success in combating Blue-green algae in the tank.

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2 thoughts on “Blue-Green Algae (Cyanobacteria) Profile and How to Remove

  1. Hi Michael, I have been looking at your various postings on snails as I have just become the proud owner of what I believe to be (from your excellent write-ups) bladder snails. It was a bit of a windy road leading up to his point. Firstly, I am an absolute amateur to this aquarium world. Secondly, I’m not even sure that what I have can be termed an ‘aquarium’. I started with a bird bath about 40cm diameter and 12 cm deep. I wanted to put a little fountain in it and enjoy the sound of water in my sunny patio. Then I discovered mosquito larvae. So I thought, why not a few mosquito-eating fish. The pet shop recommended White Clouds. So, no more mosquitoes. I made a pebble-ly underwater world for my 7 (now 6) little fish with nice big round black pebbles, and they seem pretty happy swimming in and out and around the big pebbles. But then the algae appeared because the ‘pond’ is in a rather sunny spot. Not wanting to disrupt my underwater structure with physical cleaning, and not wanting to poison my fish with chemicals, I thought I might get some algae eating critters and add them to my eco system. I found out about Nerite snails and how they don’t breed like rabbits and love to eat algae, so spent quite a few $$s and traveled to get hold of a couple of them. They did not survive 2 days in my pond. Back to the aquarium guys and they said why not some Mystery snails. So I now have a couple of very pretty, luminous snails hanging out in my pond. They do not seem to like the algae much. Again, back to the aquarium guys, this time a couple of golden algae eating fish. I never knew what they were called but it didn’t matter anyway because they disappeared from my pond the next day. I think the crow might have got them seeing how visible they were in their resplendent colours. Next, the aquarium guys got me a couple of dark coloured algae-eating fish, Bristlenose catfish. Interesting looking critters, if only I get to see more of them. They are happily ensconced somewhere underneath the pebbles and occasionally I would see a tail waving around under a rock. No algae eating action seem to be happening. Lastly came the Bladder snails. The aquarium guys actually told me they were assassin snails. My initial research had ruled that out because they did not look like assassin snail images that I find, and I did not really want them in my pond if they were actually ‘illegals’. From your descriptions, I believe they are bladder snails. I have about 10 of them happily gliding around, or sitting on the rocks, or having a bit of a ‘wrestle’ with each other, and I am worried about replacing my algae infestation with a snail infestation. And then I found another of your article here, and I quote: ‘Biological methods of control do not work with Blue-green algae’. So, I think what I might do is to fish out all the bladder snails tomorrow and then maybe buy one of the algae control products that you mentioned. I am sorry for this long ‘comment’, I guess I just wanted to thank you for your definitive posts (not just this one, but other interesting ones that have piqued my interest), and I now have my next step in my bird bath saga. The little fountain in the bird bath is no longer operating because it scares the fish.

    1. Hi Genevieve Neale,
      The aquarium hobby is quite captivating, isn’t it?
      Many started with a small tank on a shelf)) Thank you for the detailed story.
      If you have any questions, feel free to ask, and I’ll try to help.
      Best regards,

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